The present invention relates to image forming apparatuses such as copying machines, printer, fax machines and the like of the electro-photographic method, and in particular, to image forming apparatuses having a control device that can control the sheet conveying speed in the loop forming space on the upstream side of the registration device.
The registration roller of an image forming apparatus conventionally has a function of matching the timing of the sheet with the image and, at the same time, has a function of skew correction that absorbs the sheet skew generated during sheet feeding or in the conveying path. After the leading edge of the sheet is aligned by making the sheet strike against the registration roller whose drive is stopped, a bend (a loop) in the sheet in the up-down direction is generated by the sheet being conveyed for a specific time period by a loop forming roller positioned on the upstream side of the registration roller while the drive of the registration roller is stopped, and the correction of sheet skew is done by absorbing the difference between the conveyance direction towards the downstream side of the registration roller and the conveyance direction including the skew on the upstream side of the registration roller.
However, since elastic materials such as rubber are used for the registration roller and the loop forming rollers, there was the problem that, while the skew of a sheet which is long in the conveying direction is being corrected, the loop is consumed due to the distortion of the elastic material, and the ability to correct skew of the sheet is lost.
In order to solve this problem, a proposal has been made to provide a mechanism that detects the amount of loop and adjusts the speed of the feeding section that has the loop forming roller (see, for example, Patent Document 1).
However, in the above proposed mechanism, a sheet (hereinafter, also called a transfer material) that has been fed in a skewed manner gets its orientation changed in the loop forming space, the amount of loop in a direction perpendicular to the conveying direction, that is in the main scanning direction, cannot maintain uniformity. Because of this, even if the amount of loop is detected to be appropriate at a certain location in the main scanning direction, there is the possibility that the loop may not be present at one of the ends. If the amount of feed is increased uniformly so that a larger loop amount is formed considering the above, there will be problems such as buckling of the sheet or sheet folding due to excessive sheet feeding at an end where there is a larger amount of loop.    Patent Document 1: Unexamined Japanese Patent Application Publication No. Hei 6-72585.